This invention relates generally to electrical circuits for operating arc discharge flash lamps and, more particularly, to an improved operating circuit for a flash lamp directly coupled to an alternating current (AC) source.
Flashlamps of the type referred to herein generally comprise two electrodes spaced apart within an hermetically sealed glass envelope having a rare gas fill, typically xenon at a subatmospheric pressure. In typical prior art operating circuits, such lamps are connected across a large energy storage device, such as a bank of capacitors, charged to a substantial potential, but insufficient to ionize the xenon gas fill. Upon application of an additional pulse of sufficient voltage, the xenon is ionized and an electric arc is formed between the two electrodes, discharging the storage device through the flashlamp, which emits a burst of intense light. In many cases the pulse voltage is applied between an external trigger electrode, such as a wire wrapped around the envelope, and one of the internal electrodes; this is referred to as shunt triggering; however, in other cases an external wire is not feasible since it may result in undesirable arcing between the trigger wire and a proximate lamp reflector, or else the high potential applied to the external trigger wire might be hazardous to operating personnel. In those cases, the lamp may be internally triggered by applying the pulse voltage directly across the lamp electrodes, a technique referred to as injection, or series, triggering. Usually the voltage required is about 30 to 50 percent higher than that required to trigger the same lamp with an external trigger wire, and the trigger transformer secondary must carry the full lamp current.
Such flashlamps are employed in a variety of applications; for example, flash photography; reprographic machines; laser excitation; and warning flashers for airplanes, towers, road barriers, marine equipment and tower mounted approach lighting systems for airport runways. Typical prior art power supplies pose serious disadvantages for a number of these applications, however, as the required energy storage devices, such as large banks of capacitors, tend to be bulky, heavy and expensive, as are the required step-up power transformers. This is particularly apparent in endeavors to provide compact, low cost photographic flashlamps, or light weight runway flashers for mounting on frangible towers. Accordingly, it is particularly desirable to find a means for eliminating the large energy storage devices in flash lamp power supplies. In pursuit of this end, it has been observed that much higher than average short duration currents are routinely drawn from AC power lines; for example, compressor motor starting transients (locked rotor currents) are four to seven times their running currents. Metal fuses, another example, can handle peak half cycle currents of ten or more times their continuous ratings. Hence, in order to overcome the aforementioned disadvantages, it has been proposed to take advantage of this high transient current reserve capacity of conventional 120 volt, 60 Hertz AC power sources, or other commonly available lines, to draw controlled pulses of high current to operate flash lamps. Three U.S. patents that describe the direct coupling of flash lamps to an AC source are U.S. Pat. No. 3,497,768 Mathisen, U.S. Pat. No. 3,745,896 Sperti et al (FIGS. 20-25 and col. 14 on) and U.S. Pat. No. 3,896,396 Whitehouse et al. These patents are described in some detail in the background section of a copending application Ser. No. 775,122, filed Mar. 7, 1977, assigned to the same assignee as the present application.
Many xenon flash lamps, however, do not trigger well at the relatively low voltages that are encountered directly from AC service. Moreover, xenon lamps, in order to operate efficiently, must be filled to relatively high pressures (perhaps even exceeding atmospheric), a situation which further increases the triggering requirements. To assure starting lamps of relatively high fill pressure or of long arc lengths, therefore, higher voltages than are available from the line may be required. This may be true even though the lamp will operate well from the AC source at low voltages once completely ionized by triggering. The aforementioned Whitehouse et al patent counters this problem by employing a pair of capacitors across the lamp in connection with a capacitor charger to add to the current surge through the lamp during initial firing. The circuit is shown in FIG. 3 of the patent. The charger is described as including a transformer energized by a third phase of the AC source and a rectifying diode. Although providing the desired starting aid, it is apparent that the Whitehouse et al solution reintroduces into the circuitry comparatively bulky, heavy and expensive components, thereby significantly diminishing the advantages obtained by direct line coupling.